1. Field of the Invention
The present invention relates to a windshield wiper control apparatus for driving wiper blades at a wiping interval best-suited to the level of precipitation.
2. Description of the Prior Art
An automatic windshield wiper control apparatus which controls wiper operations on the basis of signals detected by a rain sensor accumulatively calculates the signals detected by the rain sensor, and initiates a wiping action when an accumulated value exceeds a certain predetermined value. Conventionally, an apparatus which calculates from the point of initiating the movement of the wiper or from the point of terminating the movement thereof has been available.
FIGS. 1(1)-1(2) are timing charts for explaining the movements of a conventional windshield wiper control apparatus which accumulatively calculates the detected signals of a rain sensor at the same time that the wiper starts. FIG. 1 (1) shows variations of the accumulated value of the detected signals with respect to time, and FIG. 1 (2) shows varations of an action position of the wiper blades with respect to time, respectively. Also, the explanation below assumes that the level of the precipitation is constant.
In the case where the accumulated value reaches the predetermined threshold value TH.sub.0 at time t11, the wiper blade which is housed in the initial position initiates the wiping action. At the same time that the wiping action the accumulated value is cleared out once. Thereafter, the detected signals of the rain sensor are accumulatively calculated. The wiper blade reaches the turning-back position at time t12 and again returns to the initial position thereof at time t13. Even through the wiper blade returns to the initial position, the detected signals of the rain sensor are further accumulatively calculated, and the accumulated value reaches the threshold value TH.sub.0 at time t14. Thus, a wiping action similar to the case mentioned above is again initiated.
FIGS. 2(1)-2(2) are timing charts for explaining the movements of another conventional windshield wiper control apparatus which calculates the detected signals of the rain sensor at the same time as terminating the wiping action. As in FIG. (1), FIG 2(1) shows variations of the accumulated value of the detected signals of the rain sensor with respect time, and FIG. 2 (2) shows variations of an action position of the wiper blade with respect to time, respectively. Also, the following assumes that the level of the precipitation is constant. The point which is different from the conventional example discussed above is that the calculation-initiating point of the detected signals of the rain sensor is from time t23 which is the point of terminating the wiping action. Namely, the wiping action starts at time t21 when the accumulated value reaches the threshold value TH.sub.0, and the accumulated value is not calculated up to the time t23 when the wiper reaches the initial position via the turning-back point at time t22. Then, the apparatus starts to calculate the detected signals of the rain sensor form the time t23, and again the wiping action initiates at time t24 when the accumulated value reaches the threshold value TH.sub.0.
On the other hand, it is optimal that the wiping action is activated at the time when raindrops attached to the exterior surface of a windshield reach a certain fixed level of precipitation and obstructs vision through the windshield. Therefore, as in the former conventional apparatus, the value calculated in the period from the t11 to the time t12 is wiped off by the wiping action in the period from the time t12 to the time t13, the wiping action to be effected from the time t14 is an unnecessary accumulated value. Therefore, since the time at which the accumulated value reaches the threshold value TH.sub.0 is faster due to the unnecessary calculation in the period from the time t11 to the time t12, a driver feels that the wiping action initiation on has occurred a little early.
The latter conventional apparatus has solved a problem which exists in the former conventional apparatus, in the point that the detected signals of the rain sensor are not calculated during the period of activation of the wiper blades from the initial position to the turning-back position like the former conventional apparatus. However, since the detected signals during the period from the time t22 to the time t23, during which the wiper blades return form the turning-back position to the initial position, are not calculated, the driver feels that the time at which the wiping action initiation has occured has been delayed. That is, it is necessary to calculate the detected signals for raindrops which attach just after a wiping action by the wiper blades in the period during which the wiper blades return from the turning-back position to the initial position.
The time at which the driver feels that he wants to activate the wiper as raindrops attach to a front windshield of a vehicle differs according to the size of the raindrops, the level of precipitation or the running speed of the vehicle. Particularly, since a drizzle, or raindrops which are splashed out by the preceding vehicle are very small in diameter, it is difficult for them to flow down after they attached to the front windshield. So, even though the amount precipitation is small, they will obstruct vision through the windshield. The larger the diameter of the raindrops becomes, the more the sensitivity of the rain sensor is heightened.
However, as discussed above, the windshield wiper control apparatuses of the prior art are composed so as to activate the wiper blades when the accumulated value obtained by calculating the output of the rain sensor exceeds a predetermined threshold value. Therefore, the wiper blades are not driven until the accumulated value exceeds the threshold value, regardless of the diameter of raindrops. For this reason, it may become difficult to secure the visibility of a driver, and the driver feels uncomfortable since there is a difference between the wiping interval desired by the driver and the actual wiping interval of the wiper blades.
Futhermore, the conventional windshield wiper control apparatuses have been composed so as to initiate the wiping action at the same time as operating the wiper switch. The wiping action (hereinafter called "initial action") which is executed at the same time as operating this wiper switch must wipe off water drops or the like instantly when a driver wishes, and this initial action is executed for the sake of making sure of whether or not the wiper can normally operate.
Hereupon, in the conventional windshield wiper control apparatuses, when a cam switch for detecting the action position of wiper blades changes, for example, from the ON mode thereof (in wiping off water drops or the like) to the OFF mode thereof (in the housing position of the wiper blades), it is judged that the initial action has been competed, and that the wiper blades have normally operated.
However, in these apparatuses, as the initial action is judged only by the changing of the cam switch from the ON mode to the OFF mode, the following inconveniences may occur. Namely, in the case where a power source to be supplied for controlling the wiper blades is turned off while the apparatus is in action, or where the wiper switch is turned on, off and on while the wiper is in action, the wiper blades do not carry out a complete wiping action, and it may be likely to only effect a part of the wiping action. Furthermore, the wiping action may be executed either on the basis of the drive signals generated from the apparatus or so as to automatically store the wiper blades in the housing portions thereof being turned since the cam switch has been kept on being turned, without depending upon control signals from the apparatus. Therefore, it is impossible to judge whether or not the wiper operates normally which is one of the objects of the initial action.
Also, in a previous windshield wiper control apparatus not having an automatic wiper function, a wiper switch for selecting the action modes of the wiper and a control circuit for effecting the wiping action in response to a setting state of the switch are arranged on a single printed circuit board and are integrally united in a body, in order to make the apparatus small in size. In order to newly furnish this apparatus with the automatic wiper function, it is necessary to provide the apparatus with a new control circuit for detecting that the wiper switch is set to the intermittent mode, and controlling the wiper operation by calculating the period of intermittence in response to the level of precipitation detected according to the output of the rain sensor.
However, in the case that the previous control circuits and the wiper switch are integrally united in a body as discussed above, it is difficult to internally detect the setting state of the wiper switch. For this reason, in the typical prior art, a switch used only for the automatic wiper function is separately added for a composition of automatic wiper operation.
Therefore, in order to effect the automatic wiper operation in the prior art discussed above it is necessary to operate the exclusive switch for a newly added automatic wiper function. Furthermore, since this added switch is arranged at a position separate from the original wiper switch, it results in a confusion and inferior operation.
Also, additionally mounting the exclusive switch for an automatic wiper function results in a necessity of modifications in the vicinity of the mounting position. Still another problem such as an unbalance between the exclusive switch for the additionally mounted automatic wiper function and the previous switch may result therefrom.